1. Field of the Invention
The invention relates to a dipole antenna useful as a radiating element in microwave and millimeter wave phased arrays, and more particularly to a printed dipole antenna with an integral balun which is useful when active circuitry is employed with each radiating element.
2. Prior Art
Dipole radiating elements with baluns for use in phased arrays have been fabricated in either a coaxial or stripline media. The coaxial versions require machined or cast metal components and either manual or specialized machine assembly. Consequently the coaxial designs tend to be relatively high in weight and cost. The coaxial dipole/balun designs require an electrical transition for interconnection to microstrip active circuitry (which has a single ground plane) and are not generally integratable with the active circuitry packaging.
Stripline dipole/balun designs, because of their printed/photolithographic fabrication process, can achieve low weight and costs. However, their double electrical ground plane complicates their utilization, and an electrical transition is required for interconnection to microstrip active circuitry (with a single ground plane) which impairs their performance. In addition, the materials usually employed for the stripline designs preclude their direct integration with the active circuitry package.
Printed microstrip "patch" type antennas are often proposed as radiating elements in active phased arrays. Patches may be directly printed with microstrip active circuitry, however, the semiconductor materials have relatively high dielectric constants which severely limit the patches' operating bandwidths. Alternatively, the patch may be integrated as part of the active circuitry package. The package materials tend to be thin and also possess high dielectric contants, both of which are detrimental to a patch's bandwidth.
A balun in a coaxial realization has been described by Roberts in an article entitled "A New Wide Band Balun," Proceedings IRE, Vol. 45, Dec. 1957, pp. 1628-1631. A printed circuit variation has been described by Bawer and Wolfe in an article entitled "A Printed Circuit Balun for Use with Spiral Antennas," IRE Trans. on Microwave Theory and Techniques, Vol. MTT-8, May 1960, pp. 319-325.
The Roberts, Bawer, and Wolfe articles describe how the balun structure can provide a broadband response when feeding a frequency independent real load. An article by Oltman entitled "The compensated Balun," IEEE Trans. on Microwave Theory and Techniques, Vol. MTT-14, March 1966, pp. 112-119, discusses the concept of selecting the characteristic impedances of the lines which comprise the balun to achieve a complementary match to a frequency dependent load impedance over a limited band.
With respect to the prior art array elements, the need has arisen for a broadband microstrip fed dipole/ balun which is light in weight, low in cost, and which can be directly interfaced with active microstrip circuitry and integrated with active circuitry packaging.